The current invention exploits a class of devices that produce spatially and/or spectrally encoded imagery or point signals by first dispersing the light from a distant scene, encoding the light using a mask that selects specific combinations of wavelengths and/or spatial positions, and then recombining the light on a detector that records the encoded polychromatic signal. Such devices often make use of a programmable spatial light modulator to create a signal-encoding mask. The programmable modulator provides the ability to encode arbitrary spatial or spectral patterns on each resolution element, and vary those patterns temporally. The patterns can be used to generate specific and highly complex spatial or spectral band passes on an image. Spectra can then be obtained by cycling the system through a sequence of spectral band passes. Spatial imagery can be obtained by encoding for different spatial elements of the dispersed image, and collected either temporally on a point detector, or simultaneously with a detector array.
Examples of such signal encoders are described in Goldstein et al. (U.S. Pat. No. 7,324,196), which describes three hardware implementations using a single point detector or detector array along with a digital micromirror device (DMD) to provide addressable spatial and spectral band selection. Sweatt et al. (U.S. Pat. No. 6,504,943) employ an input slit and linear array detectors to encode a spectral pass band on a pair of one-dimensional images. Tague (U.S. Pat. No. 5,923,036), MacKentry (1999) [MacKentry, J. W. and the NGST-MOS Study Team. “NGST-MOS A Multi-Object Spectrometer using Micro Mirror Arrays Final Report of the NGST-MOS Pre-Phase A Science Instrument Study of the NGST Project” Final Report NASA contract NAS5-98167 (1999)], Gentry (U.S. Pat. No. 6,996,292), and Fateley (U.S. Pat. No. 6,859,275) teach methods of spatial-spectral imaging in which a spatial light modulator is used to define the input slit of a spectrograph, allowing either a single detector spectrograph or a one-dimensional imaging spectrograph to select different spatial elements of an input image for spectral analysis. Fateley (U.S. Pat. No. 6,859,275) also teaches a wide variety of devices using spatial-spectral information processing.
Many of the above-mentioned signal encoders can be used to perform spatial or spectral processing in hardware. This ability, which is often referred to as compressed sensing, allows the direct measurement of a spectral contrast signal, which relates to the level of target detection confidence, with optimum duty cycle. Goldstein et al teaches the application of a spectral template designed to identify a spectral feature that is characteristic of a specific object, liquid, gas, or scene condition. Sweatt et al. teaches the application of a wide variety of analog spectral filter functions using a DMD device and dividing the light between two detection legs, each with an independently controlled amplitude modulator. Both patents are specific to applications of a defined hardware configuration. Sweatt et al. teaches the application of a variety of analog filters, including matched filters, spectral basis functions, and projection operators, by applying the positive part of the filter function to one detection leg, and the negative part of the filter function to another detection leg. The current invention allows the implementation of any analog filter, using a single detection leg and using the spatial light modulator to perform both spatial selection and amplitude modulation.